In general, inkjet printing machines or printers include at least one printhead unit that ejects drops of liquid ink onto recording media or an imaging member for later transfer to media. Different types of ink can be used in inkjet printers. In one type of inkjet printer, phase change inks are used. Phase change inks remain in the solid phase at ambient temperature, but transition to a liquid phase at an elevated temperature. The printhead unit ejects molten ink supplied to the unit onto media or an imaging member. Once the ejected ink is on media, the ink droplets solidify. The printhead unit ejects ink from a plurality of inkjet nozzles, also known as ejectors.
The media used in both direct and offset printers can be in web form. In a web printer, a continuous supply of media, typically provided in a media roller, is entrained onto rollers that are driven by motors. The motors and rollers pull the web from the supply roller through the printer to a take-up roller. The rollers are arranged along a linear media path, and the media web moves through the printer along the media path. As the media web passes through a print zone opposite the printhead or heads of the printer, the printheads eject ink onto the web. Along the feed path, tension bars or other rollers remove slack from the web so the web remains taut without breaking.
Existing web printing systems use a registration control method to control the timing of the ink ejections onto the web as the web passes the printheads. One known registration control method that can be used to operate the printheads is the single reflex method. In the single reflex method, the rotation of a single roller at or near a printhead is monitored by an encoder. The encoder can be a mechanical or electronic device that measures the angular velocity of the roller and generates a signal corresponding to the angular velocity of the roller. The angular velocity signal is processed by a controller executing programmed instructions for implementing the single reflex method to calculate the linear velocity of the web. The controller can adjust the linear web velocity calculation by using tension measurement signals generated by one or more load cells that measure the tension on the web near the roller. The controller implementing the single reflex method is configured with input/output circuitry, memory, programmed instructions, and other electronic components to calculate the linear web velocity and to generate the firing signals for the printheads in the marking stations.
Another existing registration control method that can be used to operate the printheads in a web printing system is the double reflex method. In the double reflex method, each encoder in a pair of encoders monitors one of two different rollers. One roller is positioned on the media path prior to the web reaching the printheads and the other roller is positioned on the media path after the media web passes the color printheads. The angular velocity signals generated by the two encoders for the two rollers are processed by a controller executing programmed instructions for implementing the double reflex method to calculate the linear velocity of the web at each roller and then to interpolate the linear velocity of the web at each of the printheads. These additional calculations enable better timing of the firing signals for the printheads in the marking stations and, consequently, improved registration of the images printed by the marking stations in the printing system. Ejection of ink from the inkjet nozzles can be adjusted based on the calculations. A double reflex printing system is disclosed in issued U.S. Pat. No. 7,665,817.
While control of the rotational speed of rollers is critical to the proper registration of images, other factors besides web transport can affect image registration. For instance, the material properties of the recording media can affect registration. If the continuous web slips when engaged with one or more rollers in the media path, the position of the media web with respect to the printheads is affected and errors in images formed on the media web can occur. In addition, if the web either stretches or shrinks during imaging, misregistration of images can occur. Likewise, the amount of ink deposited on the continuous web can affect the material properties of the web and result in misregistration of images as well. Consequently, improvements to a printing system and to printing images by taking into account the type of media, the amount of ink being deposited on the continuous web, and environmental conditions at the printer are desirable.